Comparison of complement fixation and microimmunofluorescence tests in respiratory infections caused by chlamydia and an evaluation of the serum amyloid A protein in chlamydial infections

We retrospectively analyzed serum samples for antibodies to chlamydia species using the microimmunofluorescence (MIF) test from 72 patients previously tested by the chlamydia complement fixation (CF) test, which is used to detect pulmonary psittacosis. Nineteen patients were positive for chlamydia with the CF test. Of these, 9 patients (47.4%) were also positive for C. psittaci infection, and 7 patients (36.8%) were positive for C. pneumoniae infection by the MIF test. Five (9.4%) and 6 (11.3%) of the 53 CF-negative patients were positive for psittacosis and C. pneumoniae infection by the MIF test, respectively. Our results indicate that the serum of patients suspected of pulmonary psittacosis should be examined by the MIF test as well as CF in order to distinguish between infections caused by C. psittaci, C. pneumoniae and C. trachomatis, and to improve the sensitivity of serodiagnosis. Serum samples of all 7 patients positive by both CF and the MIF test for C. pneumoniae infection showed a diagnostic IgG antibody change to C. pneumoniae without IgM antibody changes, which suggested a primary infection. These results suggest that the diagnostic CF antibody response occasionally appears in reinfection, especially in C. pneumoniae infection. There was a positive correlation (r= 0.858) between C-reactive protein and serum amyloid protein A, a sensitive acute-phase serum reactant, over a wide range of concentrations in 19 patients with chlamydia respiratory infections.